Interferometry is widely used to measure changes in distance and to measure the topology of a surface at the micro level. The latter application is often referred to as surface profiling. The general principle, illustrated with reference to a conventional interferometry system 100 (see FIG. 1), involves splitting a beam 11 of light into two portions 12 and 15, reflecting one portion 12 of the beam 11 from a reference surface 13 and the other portion 15 from a measured surface 16, combining the two portions 12, 15 of the beam 11 into a single beam 18, and detecting the combined, single beam 18 via an optical detector 17. Beams of light illustrated in this and the other drawing figures herein are shown as double lines, signifying the outside opposing edges of the diameter of the beam. The beam that starts from the light source 10 and is reflected from the reference surface 13 and passes to the optical detector 17 is the reference beam, and the beam that starts from the light source 10, is reflected from the measured surface 16 and passes to the optical detector 17 is the measurement beam.
When the optical path length of the reference beam is equal to the optical path length of the measurement beam, then the two beams interfere constructively. If single wavelength light is used and the paths along which the measurement beam and the reference beam travel differ by half a wavelength, then the two beams of light interfere destructively, and the optical detector 17 detects a signal of minimum amplitude. Similarly, whenever the difference between the two paths is n*(λ/2), where n is an odd integer and λrepresents the wavelength of the light, the optical detector 17 again detects a signal of minimum level, and when the difference between the two paths is m*(λ/2), where m is an even integer, the optical detector 17 detects a maximum signal. If the object being measured (e.g., the measured surface 16) moves half a wavelength toward or away from the beam splitter 21, the path length of the measurement beam will change by one complete wavelength, and the optical detector 17 will go through one complete cycle of intensity detected by the optical detector 17. A region in which the combined intensity of the reference beam and the measurement beam is at a minimum is referred to as a fringe. Quarter wave plates 22 and 23, shown in FIG. 1, may be optionally inserted in the reference beam path and the measurement beam paths, respectively, to reduce errors caused by reflections, as is known in the art.
When a single optical detector is used to detect the average intensity of beam 18, the optical detector measures the difference in path length between the measurement beam and the reference beam. This configuration is useful to detect changes in distance between the measured surface 16 and the beam splitter 21. If an optical detector with a two dimensional array of optical detecting elements, such as a CCD camera, is used, and the diameter of the light beam is configured to be large enough to illuminate the complete two-dimensional optical detector, then each element of the optical detector acts as a separate optical detector, and the system functions as multiple interferometers operating in parallel. The area of each optical detector element creates a pixel, a word which is a contraction of the words “picture” and “element”. When the measured surface is not perfectly smooth, there will be different optical path lengths for different pixels, causing phase differences between the signals at different optical detector elements and therefore different intensity signals at each detector element. The difference in intensity at different optical detector elements can be converted to phase differences, and the phase differences can be converted to distances, yielding a three-dimensional map of the topology of the area seen by the complete two-dimensional optical detector.
While multiple phase calculation methods exist, in general, a complete measurement requires moving the reference surface in multiple discrete increments to capture fringe pattern images at each position of the reference mirror while the measured surface does not move. Once these images are captured the data of the multiple frames are used to calculate the phase information at the corresponding pixels. This surface profiling technique, in which the reference surface moves in multiple discrete steps, is referred to as Phase Shifting Interferometry and equipment using this technique is referred to as a Phase Shifting Interferometer (PSI).
Phase Shifting Interferometers cannot determine a step height with certainty if the height changes instantaneously between neighboring pixels by more than plus or minus λ/2, because a PSI using single wavelength light cannot distinguish between a phase change of ΔΦ and a phase change of ΔΦ+nλ, where ΔΦ is the phase difference between the reference beam and the measurement beam and n is an integer. Following a similar principle, in order to measure a surface whose height is changing relatively rapidly from one pixel to another, single wavelength PSIs increment the movement of the reference surface (or the measured surface) by an amount which is less than λ/2 and assume that n, in the expression ΔΦ+nλ/2, is zero. Moving the reference surface through a fixed distance in small discrete increments and collecting intensity signal information at each position of the reference surface requires considerably more measurement time than acquiring intensity data at a single position of the reference surface.
While interferometers using multiple wavelengths of light, or even white light, are better at measuring step height than single wavelength PSIs, such interferometers require moving the reference surface in discrete increments over a distance far greater than λ/2, where λ is the wavelength used for a single wavelength interferometer, requiring additional measurement time.
The lateral resolution of such a surface profiler is a function of both the size of the elements in the optical detector and of the optics which image the measured surface onto the optical detector elements. In order to obtain better lateral resolution, one uses greater magnification, resulting in a measurement of a smaller area of the measured surface. The software of typical Phase Shifting Interferometer systems can stitch together multiple images, taken by measuring one site, moving the measured surface to another site, measuring at that site, etc., but taking multiple images requires even more time. Further, the assumption behind stitching is that there is no system drift between adjacent images, producing no discontinuities. Thermal drift and vibration can create errors in the stitched images. Errors caused by stitching and the large amount of time required to make measurements of suitable lateral and vertical resolution often make PSIs unsuitable for quality control in a production environment.
Phase Shifting Interferometer measurements often suffer from error sources such as inaccurate knowledge of the exact position of the reference mirror and inaccurate positioning of the intended discrete positions of the reference mirror. Further PSI interferometers cannot distinguish between vibration of the measured surface, such as might be caused by sound waves impinging on the measured surface, from changes in the actual roughness of the measured surface.
It would therefore be desirable to have interferometry systems and methods that avoid at least some of the drawbacks of the conventional interferometry systems and methods described above.